The present invention generally relates to helical inductors, particularly resonator helical filters and more particularly to RF helical filter arrangements formed using printed circuit board techniques.
RF helical filter apparatus incorporating one or more helical resonators are known in the art. Typically, these filter arrangements include a plurality of resonators constructed from conductors forming helical coils. The resonator coils encircle a cylindrical space. The helical coils are mounted on a suitable base plate which spacedly disposes the resonator helical coils. The coils themselves are connected to electrical ground at one end and a signal tap lead to connected to the helical coil at a selected tap point. Frequently, a tuning slug is provided to change the electrical characteristics of the coil to tune the filter. When provided, the tuning slug is generally threadably received in the upper portion of the helical coil distal from the mounting plate. The tuning slug is rotated to axially displace the tuning slug with respect to the helical coil to change the tuning of the coil.
The coils themselves are formed from a wire material to extend in a helical fashion. Where a tuning slug is used, the tuning slug is dimensioned to be received in the cavity or bore centrally formed by the helically disposed wire coil. Commonly, helical filter arrangements have three resonator coils encased within a shielding case or can to contain the radio frequency (RF) signal radiation emanating from each coil. The shielding case includes a baffle structure forming a contiguous box structure to encase each resonator coil separately. The baffle structure provides an aperture in the common wall of the box cavities encasing each of the separate resonator coils to permit RF coupling to occur between adjacent resonator coils. The shape and size of the aperture is configured to control the RF coupling between each coil. In a three coil filter arrangement, the central resonator is RF coupled to two end resonators by apertures in the common side walls contained within the overall shielding can that encloses the resonator apparatus. The end resonators are connected to a tap lead that extends from the helical coil to a signal conductor trace formed on the printed circuit board to supply signalling to or receive signalling from the resonator filter. Each tap point associated with a helical resonator is selected to provide an impedance match to the circuit to which the filter arrangement is to be electrically connected.
Heretofore, construction of resonator filter helical coils required the supply of a relatively large gauge wire which is wrapped in a helix forming device and then cut to form the helical resonator. Such a construction method has several disadvantages including the need to provide wire of a structural dimension to form the helical resonator. Also, forming equipment is needed to wrap and bend the wire and cut and configure it into a helical structure for use in the resonator.
The complexity and cost of producing printed circuit board assemblies incorporating RF helical filter resonators could be reduced if the need for relatively large gauge formed wire could be eliminated. Furthermore, the cost of producing printed circuit board assemblies that include RF helical filter resonators could be reduced if the need for wire forming equipment to form wire helix structures could be eliminated. Accordingly, there exists the need for construction of helical resonators which do not require the supply of wire of a relatively large dimension to form the structure of helical coils. Moreover, there exists a need to fabricate helical coils without the requirement to have wire forming apparatus to bend wire into a helical structure to form the resonator.